Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i, comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i, further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.
When seeking the head across the disk, a servo fault may occur due to loss of synchronization to the servo sectors, a large deviation in the estimated servo states of the servo controller, detecting instability of the servo controller, etc. When a servo fault is detected, the prior art quickly decelerates the head to zero velocity using, for example, a double integrator model-based open loop control. After the head reaches zero velocity, a state estimator is re-qualified based on the servo sectors, and the seek is completed.
FIG. 4A illustrates an example velocity/position phase plane for a seek operation executed by a prior art servo controller. In this example, the seek is performed using a just-in-time (JIT) control which typically reduces acoustic noise as compared to a minimum-time seek control. If a servo fault does not occur, the servo states will follow the arcuate trajectory 15 shown in FIG. 1. If a servo fault occurs, for example, when the head reaches position 17, the servo controller quickly decelerates the head to zero using a minimum-time seek control such that the servo states follow the steep deceleration trajectory 19. After the head reaches zero velocity, the servo controller re-qualifies the state estimator, and then completes the seek after accelerating to a relatively low coast velocity 21. Decelerating the head to zero velocity in minimum time when a servo fault is detected such that the servo states follow a steep deceleration trajectory may excite resonances in the servo controller which can increase acoustic noise, or cause other issues, such as poor seek settle, command time-out, or even an off-track write. In addition, a significant error in the estimated servo states used to initialize the double integrator model-based open loop control may cause a high-speed runaway condition which may damage the head due to the actuator arm colliding with a crash stop.